Diffuse-reflection-based single-ended laser absorption sensor for H₂O temperature and concentration in kerosene-fuelled combustor

Abstract

The design, optimization and demonstration of a compact, single-ended laser absorption sensor based on a diffuse-wall-reflected signal is presented for temperature and H 2 O concentration measurements in a kerosene-fuelled aero-combustor. The challenges of laser measurements in such harsh, practical combustion environments involve strong beam-steering, low signal-to-noise ratio (SNR), and constrained optical access. We present here a detailed characterization and optimization procedure of a single-ended optical configuration using a holed off-axis parabolic mirror as the pitch and catch optics, and the coked wall surface of the combustor as a diffuse reflector. A near-infrared distributed-feedback diode laser near 1.4 µm scanned at 1 kHz for direct absorption detection was used to access two H 2 O transitions for concentration and temperature measurement. An optical system design and adaptive Savitzky-Golay signal processing algorithm were used for noise suppression and SNR amelioration. The sensor was demonstrated under practical conditions in a kerosene-fuelled swirl combustor. High signal fidelity was achieved and allowed for a single-scan H 2 O concentration detection limit of 122 ppm MHz −1/2 , despite a low reflected laser intensity level of ~50 µW. In situ, time-resolved measurements detected temperatures ranging from ~1100 K to 1300 K, and H 2 O concentration ranging from ~8% to 12% as global equivalence ratio varied between 0.2 and 0.45. The results revealed an expected trend while demonstrating much faster response and less delay versus a comparative thermocouple. This successful field manifestation provides direct proof for the robustness and reliability of a diffuse-reflection-based single-ended sensor system in engine combustion chamber environments.